Abstract: A fish cage is for farming of fish, the fish cage having one of a water-tight outer wall and a close to water-tight outer wall and the outer wall is form stable, an entrance for water into the fish cage and an outlet for water out of the fish cage at a lower portion. The fish cage is adapted for an active water flow from an inlet at an upper portion through the fish cage to the outlet, and the fish cage is adapted to keep a shoal of fish with a negative buoyancy in a region within the water flow, such that the shoal of fish creates the water flow. A method is for exchanging water in a fish cage by utilizing the fish is also described.

Abstract: A floating fish farming plant including a cage supported by a buoyant body which is configured for floating in a body of water. The farming plant further includes a lifting portion arranged between the buoyant body and the cage and configured for moving the cage between a position at the level of, or below, the buoyant body and a position in which at least a portion of the cage is above the buoyant body. The cage includes at least a lower section, forming a bottom portion of the cage, said lower section having a wall comprising an elastic material and/or being elastically connected to the middle sections; whereby transfer of dynamic forces between the lower section and the adjoining section is reduced. An assembly comprises a plurality of fish farming plants.